1. Field of the Invention
The present invention relates to a novel copolymer, to a photoresist composition and to a process for the formation of a resist pattern with a high aspect ratio. Specifically, the present invention relates to a photoresist composition that is excellent in transparency to light with wavelengths in the deep ultraviolet region (deep UV light) such as KrF or ArF laser light, and exhibits high sensitivity and definition and satisfactory depth of focus (DOF) properties, to a novel copolymer that is suitable for the preparation of the photoresist composition, and to a process for the formation of a resist pattern with a high aspect ratio.
Additionally, the present invention relates to a chemically amplified photoresist composition that is suitable for a process for the formation of a resist pattern with a high aspect ratio using a silylation agent, and to a process for the formation of a resist pattern with a high aspect ratio.
2. Description of the Related Art
To form an ultrafine resist pattern of not more than 0.35 xcexcm, specifically of not more than 0.25 xcexcm, a variety of highly sensitive photoresist compositions having satisfactory transparency to deep UV light such as KrF or ArF laser light have been reported.
Among them, xe2x80x9cchemically amplifiedxe2x80x9d photoresist compositions containing a substance carrying an acid-decomposable group, and a photosensitive acid generator are predominant and have been reported as follows.
Japanese Patent Laid-Open No. 5-11450 (Reference 1) discloses, as the aforementioned photoresist compositions, a photoresist composition containing a polymer and a photosensitive acid generator, which polymer comprises a (meth)acrylic tert-butyl ester group and a maleic anhydride functional group, and a photoresist composition containing a polymer and a photosensitive acid generator, which polymer comprises a (meth)acrylic tert-butyl ester group, a maleic anhydride functional group, and an allyltrimethylsilane group.
These compositions have satisfactory transparency to deep UV light and have high sensitivity and definition, but their DOF properties must be further improved. Specifically, demands have been made to broaden acceptable DOF ranges in the formation of a hole pattern with a hole diameter of not more than 0.35 xcexcm.
Japanese Patent Laid-Open No. 11-212265 (Reference 2) discloses a photoresist composition comprising a polymer and a photosensitive acid generator, which polymer contains a repeating unit represented by Formula (III) and a repeating unit represented by Formula (VII): wherein R1 is a hydrogen atom or a methyl group; and R2 is an alkyl group having from 1 to 4 carbon atoms. 
This composition has satisfactory transparency to deep UV light and has high sensitivity and definition, but its DOF properties must be further improved. Specifically, demands have been made to broaden acceptable DOF ranges in the formation of a hole pattern with a hole diameter of not more than 0.35 xcexcm.
As a possible means to form an ultrafine resist pattern of not more than 0.35 xcexcm, and specifically of not more than 0.25 xcexcm, a process using a silylation agent is reported in, for example, Microelectronic Engineering 11 (1990) 531-534 (Reference 3).
As is described in Reference 3, the process for the formation of a resist pattern with a high aspect ratio using a silylation agent is performed substantially in the following manner.
Initially, a resist film (bottom resist) is formed on a substrate, and this bottom resist is resistant to etching for the substrate. Another resist film is then formed on the bottom resist, using a photoresist composition containing, for example, a polymer having a repeating unit represented by Formula (V) below, and the formed resist film is selectively patterned by exposure and developing to thereby form a resist pattern.
Next, a solution of, for example, a compound represented by Formula (VIII) below (silylation agent) is applied onto the resist pattern and is then rinsed to thereby form a silylation coating on the resist pattern, and the resulting silylation coating is resistant to corrosion induced by oxygen plasma etching.
The underlying bottom resist is removed by etching using, as a mask, the patterned resist carrying the silylation coating to thereby form a resist pattern with a high aspect ratio. 
Such resist compositions that can be applied to this process must react with a silylation agent to thereby form a silylation coating and must have satisfactory DOF properties in addition to high sensitivity and definition.
Accordingly, an object of the present invention is to provide a chemically amplified photoresist composition that has satisfactory transparency, high sensitivity and definition and exhibits satisfactory DOF properties in the field of photolithography using a deep UV light source such as KrF or ArF laser, and to provide a novel copolymer that is suitable for the preparation of the photoresist composition.
Another object of the present invention is to provide a chemically amplified photoresist composition that is suitable for a process for the formation of a resist pattern with a high aspect ratio using a silylation agent, and to provide a process for the formation of a resist pattern with a high aspect ratio.
After intensive investigations, the present inventors have found that the above objects can be achieved by the use of novel copolymers having specific repeating units.
Specifically, the present invention provides, in an aspect, a novel copolymer including a repeating unit (A) represented by Formula (I) below; and a repeating unit (B) derived from an unsaturated carboxylic anhydride: 
wherein R1 is a hydrogen atom or a methyl group; and R2 is an alkyl group having from 1 to 4 carbon atoms.
The novel copolymer preferably further includes a repeating unit (C) represented by Formula (II): 
The novel copolymer preferably further includes a repeating unit (D) represented by Formula (III): 
wherein R1 and R2 have the same meanings as defined above.
The repeating unit (A) in the novel copolymer is preferably a unit (A-1) represented by Formula (IV): 
The repeating unit (B) in the novel copolymer is preferably a repeating unit (B-1) derived from an unsaturated cyclic carboxylic anhydride.
The repeating unit (B-1) is preferably a unit (B-2) represented by Formula (V): 
Preferably, the repeating unit (D) in the novel copolymer is a unit (D-1) represented by Formula (VI): 
The content of the repeating unit (A) in the novel copolymer is preferably more than 0% and less than or equal to 40% of all repeating units constituting the novel copolymer.
The content of the repeating unit (B) in the novel copolymer is preferably equal to or more than 15% and less than or equal to 60% of all repeating units constituting the novel copolymer.
The content of the repeating unit (C), if any, in the novel copolymer is preferably equal to or more than 10% and less than or equal to 40% of all repeating units constituting the novel copolymer.
The content of the repeating unit (D), if any, in the novel copolymer is preferably more than 0% and less than or equal to 40% of all repeating units constituting the novel copolymer.
The novel copolymer preferably has a weight average molecular weight (Mw) in terms of polystyrene of from 7000 to 30000 and a molecular-weight distribution (Mw/Mn, where Mn is a number average molecular weight) of less than or equal to 3.5.
In another aspect, the present invention provides a photoresist composition including the novel copolymer, a photosensitive acid generator, and an organic solvent.
In the photoresist composition, the photosensitive acid generator is preferably a triphenylsulfonium-based onium salt.
The organic solvent is preferably propylene glycol monomethyl ether acetate (PGMEA).
In addition and advantageously, the present invention provides a process for forming a resist pattern with a high aspect ratio, which process includes the steps of (a) applying a first resist on a substrate and drying the applied first resist to thereby form a first resist layer, applying the photoresist composition onto the first resist layer and drying the applied photoresist composition to thereby form a second resist layer; (b) exposing the second resist layer to imaging radiation, subjecting the exposed second resist layer to a heat treatment, and dissolving and removing exposed portions or unexposed portions of the second resist layer by developing in an alkaline aqueous solution to thereby form a resist pattern; (c) applying a silylation agent onto the formed resist pattern and rinsing the resist pattern to thereby enlarge the resist pattern and to form a silylation coating on the resist pattern, which silylation coating is resistant to corrosion induced by oxygen-containing plasma etching; and (d) etching the underlying first resist layer with oxygen-containing plasma by using, as a mask, the enlarged resist pattern carrying the silylation coating.